This invention relates to a silver halide photographic element with enhanced light absorption and low dye stain. It more specifically relates to a silver halide photographic element containing a dye layered emulsion and a specific class of sensitizing dyes.
J-aggregating cyanine dyes are used in many photographic systems. It is believed that these dyes adsorb to a silver halide emulsion and pack together on their xe2x80x9cedgexe2x80x9d which allows the maximum number of dye molecules to be placed on the surface. However, a monolayer of dye, even one with as high an extinction coefficient as a J-aggregated cyanine dye, absorbs only a small fraction of the light impinging on it per unit area. The advent of tabular emulsions allowed more dye to be put on the grains due to the increased surface area per mole of silver. However, in most photographic systems, it is still the case that not all of the available light is being collected.
The need is especially great in the blue spectral region where a combination of low source intensity and relatively low dye extinction results in a deficient photo response. The need for increased light absorption is also great in the green sensitization of the magenta record of multilayer color film photographic elements. The eye is most sensitive to the magenta image dye, and this layer has the largest impact on color reproduction. Higher speed in this layer can be used to obtain improved color and image quality characteristics. The cyan layer could also benefit from increased red-light absorption that could allow the use of smaller emulsions with less radiation sensitivity and improved color and image quality characteristics. For certain applications, it may be useful to enhance infrared light absorption in infrared sensitized photographic elements to achieve greater sensitivity and image quality characteristics.
One way to achieve greater light absorption is to increase the amount of spectral sensitizing dye associated with the individual grains beyond monolayer coverage of dye (some proposed approaches are described in the literature, G. R. Bird, Photogr. Sci. Eng., 18, 562 (1974)). One method is to synthesize molecules in which two dye chromophores are covalently connected by a linking group (see U.S. Pat. Nos. 2,518,731; 3,976,493; 3,976,640; and 3,622,316; Kokal Sho 64(1989)91134, and EP 565 074). This approach suffers from the fact that when the two dyes are connected, they can interfere with each other""s performance, e.g., not aggregating on or adsorbing to the silver halide grain properly.
In a similar approach, several dye polymers were synthesized in which cyanine dyes were tethered to poly-L-lysine (U.S. Pat. No. 4,950,587). These polymers could be combined with a silver halide emulsion; however, they tended to sensitize poorly, and dye stain (an unwanted increase in D-min due to retained sensitizing dye after processing) was severe in this system and unacceptable.
A different strategy involves the use of two dyes that are not covalently linked to one another. In this approach the dyes can be added sequentially and are less likely to interfere with each other. Miyasaka et al. in EP 270 079 and EP 270 082 describe silver halide photographic material having an emulsion spectrally sensitized with an adsorbable sensitizing dye used in combination with a non-adsorbable luminescent dye that is located in the gelatin phase of the element. Steiger et al. in U.S. Pat. Nos. 4,040,825 and 4,138,551 describe a silver halide photographic material having an emulsion spectrally sensitized with an adsorbable sensitizing dye used in combination with a second dye that is bonded to gelatin. The problem with these approaches is that unless the dye that is not adsorbed to the grain is in close proximity to the dye adsorbed on the grain (less than 50 angstroms separation), efficient energy transfer will not occur (see T. Fxc3x6rster, Disc. Faraday Soc., 27, 7 (1959)). Most dye off-the-grain in these systems will not be close enough to the silver halide grain for energy transfer, but will instead absorb light and act as a filter dye leading to a speed loss. A good analysis of the problem with this approach is given by Steiger et al. (Photogr. Sci. Eng., 27, 59 (1983)).
A more useful method is to have two or more dyes form layers on the silver halide grain. Penner and Gilman described the occurrence of greater than monolayer levels of cyanine dye on emulsion grains, Photogr. Sci. Eng., 20, 97 (1976); see also Penner, Photogr. Sci. Eng., 21, 32 (1977). In these cases, the outer dye layer absorbed light at a longer wavelength than the inner dye layer (the layer adsorbed to the silver halide grain). Bird et al. in U.S. Pat. No. 3,622,316 describes a similar system. A requirement was that the outer dye layer absorb light at a shorter wavelength than the inner layer. A problem with previous dye layering approaches was that the dye layers described produced a very broad sensitization envelope. This may be desirable for some black-and-white photographic applications, but in a multilayer color film element this would lead to poor color reproduction since, for example, the silver halide grains in the same color record would be sensitive to both green and red light.
Yamashita et al. (EP 838 719 A2, U.S. Pat. No. 6,117,629) describes the use of two or more cyanine dyes to form more than one dye layer on silver halide emulsions. The dyes are required to have at least one aromatic or heteroaromatic substituent attached to the chromophore via the nitrogen atoms of the dye. Yamashita et al. teaches that dye layering will not occur if this requirement is not met. This is undesirable because such substitutents can lead to large amounts of retained dye after processing (dye stain) that affords increased D-min. Similar results are described in U.S. Pat. No. 6,048,681. EP 1 061 411 A1 describes forming dye layers by using dyes with additional polycyclic rings. The dyes have at least one heterocyclic ring that has two or more additional rings attached to it. This may promote dye-dye interactions by increasing van der Waals forces; however, adding hydrophobic, aromatic rings to the dye molecules is undesirable in that the dyes are more likely to be retained after processing and give higher dye stain. Yamashita and Kobayashi (JP 10/171058) describe silver halide photographic emulsions that contain an anionic dye and a cationic dye, where the charge of either the anionic dye or the cationic dye is two or greater.
Further improvements in dye layering have been described in U.S. Pat. Nos. 6,143,486; 6,165,703; 6,329,133; 6,331,385; and 6,361,932. Useful antenna dyes for dye layering that have less dye stain after processing were described in U.S. Pat. No. 6,312,883.
However, even with the improvements in dye layering technology to date, it is still difficult to use emulsions with more than one layer of dye in a multilayer color film element that generally carries a higher coated level of silver halide. The reason is that the level of stain from the excess dye from the second layer is often unacceptable and contributes to the minimum density. This is particularly true for the green and red density of color negative and color reversal films. In color reversal films, the retained dye is visually noticeable and objectionable. In color negative films, higher levels of green and red density have a negative impact on printing because a higher density in the negative requires a longer or more intense exposure when printing the negative. This is a particular problem for the green density of color negative films. Red sensitive emulsions in color negative films are commonly dyed with J-aggregating cyanine dyes, but when these dyes are retained in the film, they are present in the monomeric state where they absorb green light. Thus, retained red dyes add to green density that is already high from the presence of magenta colored cyan image couplers and the presence of retained green sensitizing dyes. Dyes that are useful for forming a second layer on red sensitive emulsions, i.e., they absorb red light, will also contribute to additional green minimum density after processing, raising the green density to unacceptable levels.
A particular problem with retained green sensitizing dye is the impact of the dye absorption on automatic printers. An automatic printer measures the green density through a narrow filter that has a peak transmission at 525 nm or longer wavelength. The printer measures the minimum blue, green, and red density through separate filters and sets the appropriate exposure for the negative based on the measured values. A green sensitizing dye or antenna dye that is retained in its monomeric state will have a peak absorption in the film around 500 to 510 nm. The filter in the printer will not measure the density contribution of the retained dye correctly because the filter is measuring transmitted light at a longer wavelength. It will set the printer for a green exposure on the paper that is too low. However, the green sensitivity of the paper between 500 and 510 nm is substantial, and the added density of the retained monomeric green dye in the negative will cause the paper to be underexposed with green light. This means that a film incorporating a green antenna dye that results in a higher absorption of light around 500 to 510 nm will not print compatibly with a similar film that does not contain a green antenna dye. This is known in the trade as poor printer compatibility and leads to lower quality prints or added time and expense for the photofinisher. It is undesirable and must be remedied for the practical use of antenna dyes in multilayer consumer color negative films.
The use of more than one dye layer on silver halide emulsions to enhance light absorption in a multilayer photographic element is often accompanied by unacceptably high levels of post-process retained dye (dye stain). This dye stain can lead to poor printer compatibility that produces lower quality prints and added cost to the photofinisher. It would be highly desirable if combinations of dyes could be found that allow the use of more than one layer of dye on the emulsion surface without a large, unacceptable increase in dye stain.
This invention provides a silver halide photographic element comprising at least one silver halide emulsion comprising silver halide grains which have associated therewith at least an inner dye layer and an outer dye layer wherein the outer dye layer comprises a dye having at least one substituent that has a positive charge, said photographic element further comprising a cyanine dye of formula I or II that is capable of spectrally sensitizing a silver halide emulsion: 
wherein:
X is O or NR4;
Y is O, S, or NR4;
R1 is H or a 1-4 carbon alkyl group;
R2 and R3 are independently a 1-6 carbon alkyl group comprising an acid salt substituent;
R4 is a 1-4 carbon alkyl group;
V1 to V4 are hydrogen or substituents with a pi constant of less than 1.0;
M is a counterion to balance the charge if necessary; 
wherein:
W is O, S, Se, or NR4;
Z is S or Se;
R5 is H or a 1-4 carbon alkyl group;
R6 and R7 are independently 1-6 carbon alkyl groups comprising an acid salt substituent;
R4 is a 1-4 carbon alkyl group;
V5 to V8 are hydrogen or substituents with a pi constant of less than 0.75 provided that at least three of V5 to V8 are hydrogen or substituents with a pi constant of 0.65 or less;
M1 is a counterion to balance the charge if necessary.
This invention provides a photographic element having the advantages of dye layered emulsions without a large, unacceptable increase in dye stain.